Worker wrongly used hydrogen peroxide, which neutralised the chlorine.

After a week of trying to part with green tides in two outdoor swimming pools, Olympic officials over the weekend wrung out a fresh mea culpa and yet another explanation—neither of which were comforting.

According to officials, a local pool-maintenance worker mistakenly added 160 liters of hydrogen peroxide to the waters on August 5, which partially neutralized the chlorine used for disinfection. With chlorine disarmed, the officials said that “organic compounds”—i.e. algae and other microbes—were able to grow and turn the water a murky green in the subsequent days.

The revelation appears to contradict officials’ previous assurances that despite the emerald hue, which first appeared Tuesday, the waters were safe.

“Of course it’s an embarrassment,” Gustavo Nascimento, director of venue management for the Rio Olympics, told the New York Times. “We are hosting the Olympic Games, and athletes are here, so water is going to be an issue. We should have been better in fixing it quickly. We learned painful lessons the hard way.”

Nascimento said it took a while to figure out what happened because hydrogen peroxide wasn’t supposed to be used at all, and it was not detected by initial tests. “The electronic monitoring system that measures the amount of chlorine in the water was betrayed by this chemistry,” he said. Officials had previously blamed the pools’ swamp-like shade on algae, people, and a shortage of an unnamed chemical.

Further Reading

Hydrogen peroxide is sometimes used in pools—often to de-chlorinate them. Basically, the chemical, a common household disinfectant, is a weak acid that reacts with chlorine and chlorine-containing compounds to release oxygen and form other chlorine-containing compounds. Those may not be good at disinfecting pools, but they still may be picked up by monitoring systems.

Hydrogen peroxide can also be used to disinfect pools but must be maintained in the waters—not a one-time dumping—and can’t be used in combination with chlorine.

On Saturday, officials started draining and refilling one of the affected pools—the one used for synchronized swimming, a sport that requires underwater visibility. The 3,725,000-liter pool was refilled with water from a clean practice pool nearby. The diving pool, the first to turn green, is being filtered and treated to clean the waters.

By the end of last week, athletes and media reported that the waters had begun to irritate eyes and smell like farts.